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Using a different domain name for Mastodon and the users it serves

Mastodon users are primarily identified by a user@example.org handle, and you might want this identifier to be the same as your email or jabber account, for instance. However, in this case, you are almost certainly serving some web content on https://example.org already, and you might want to use another domain (say social.example.org) for Mastodon itself.

Luckily, there is support in Mastodon for that, but it might be tricky to set up, and any error might prevent you from federating with other instances.

This guide attempts to explain the different mechanisms involved in such a setup, walk you through the configuration, and warn you about possible federation issues. It also assumes you have already read the Production guide.

Account identifiers

It is important to understand that for federation purposes, a user in Mastodon has two unique identifiers associated:

  • A webfinger acct: URI, used for discovery and as a verifiable global name for the user across Mastodon instances. In our example, our account's acct: URI is acct:user@example.org
  • An author/actor URI, used in every other aspect of federation. This is the way in which users are identified in OStatus, the underlying protocol used for federation with other Mastodon instances. In our case, it is https://social.example.org/users/user. It is not strictly required to be an URL, but they are in Mastodon's case to ease discovery of unknown remote accounts.

Both account identifiers are unique and required for Mastodon. An important risk if you set up your Mastodon instance incorrectly is to create two users (with different acct: URIs) with conflicting author/actor URIs.

Webfinger

As said earlier, each Mastodon user has an acct: URI, which is used for discovery and authentication. When you add @user@example.org, a webfinger query is performed. This is done in two steps:

  1. Querying https://example.org/.well-known/host-meta (where the domain of the URL matches the domain part of the acct: URI) to get information on how to perform the query. The host-meta file should contain a URL template of the form https://somedomain.org/.well-known/webfinger?resource={uri} that will be used in the second step.
  2. Fill the returned template with the acct: URI to be queried and perform the query: https://somedomain.org/.well-known/webfinger?resource=acct:user@example.org

Configuring your Mastodon instance

DO NOT ATTEMPT TO CONFIGURE YOUR INSTANCE THIS WAY IF YOU DID NOT UNDERSTAND THE ABOVE

Also, make sure to check the known issues below.

Configuring Mastodon itself

Mastodon has a two configuration variables in .env.production to enable using different domains for your users and Mastodon itself. Those two variables are LOCAL_DOMAIN and WEB_DOMAIN. When the latter is not set, it defaults to the value of LOCAL_DOMAIN.

Be extra careful when configuring your Mastodon instance, as changing LOCAL_DOMAIN without changing WEB_DOMAIN may cause remote instances to register different accounts with the same author/actor URI, which will result in federation issues!

  • LOCAL_DOMAIN is the domain for which your Mastodon instance has authority, it is the domain used in the acct: URI. In our example, LOCAL_DOMAIN would be set to example.org.
  • WEB_DOMAIN is the domain used for any URL generated for your instance, including the author/actor URIs. In our case, that would be social.example.org.

Configuring example.org

Now, you have Mastodon running at https://social.example.org as well as a website at https://example.org. If you recall how webfinger queries work, the first step is to query https://example.org/.well-known/host-meta, which will contain an URL template.

Therefore, the easiest way to configure domain.org is to redirect /.well-known/host-meta to social.example.org.

With nginx, it would be as simple as adding:

location = /.well-known/host-meta {
       return 301 https://social.example.org$request_uri;
}

in example.org's server block.

Alternatively, you can serve the file from your hosting. The full content of the file:

<?xml version="1.0" encoding="UTF-8"?>
<XRD xmlns="http://docs.oasis-open.org/ns/xri/xrd-1.0">
  <Link rel="lrdd" type="application/xrd+xml" template="https://social.example.org/.well-known/webfinger?resource={uri}"/>
</XRD>

Known issues

There are a few known issues with Mastodon:

  • Current mastodon instances won't correctly process inbound salmon requests from instances where WEB_DOMAIN!= LOCAL_DOMAIN, and will try resolving acct:user@WEB_DOMAIN instead of acct:user@LOCAL_DOMAIN. Fortunately, since v1.3.0, Mastodon will reply to acct:user@WEB_DOMAIN queries with the account informations for acct:user@LOCAL_DOMAIN, effectively working around this issue at the cost of an extra webfinger discovery per interaction.
  • Mastodon does not actually use WEB_DOMAIN everywhere it should and will instead use whatever host it was accessed with. For this reason, your Mastodon instance should only be reachable on WEB_DOMAIN and not LOCAL_DOMAIN (HTTP redirects are fine, but avoid proxying from LOCAL_DOMAIN to WEB_DOMAIN)
  • Remote Mastodon instances on v1.3.0, v1.3.1 or v1.3.2 from which you are following people won't PuSH new messages to your instance. This is a known bug, see #2672